[R] Deleting many list elements in one time

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Duncan Murdoch
> Sent: Monday, April 05, 2010 8:24 AM
> To: anna
> Cc: r-help at r-project.org
> Subject: Re: [R] Deleting many list elements in one time
> 
> On 05/04/2010 11:17 AM, anna wrote:
> > Hi guys, here is a simple thing I want to do but it doesn't work:
> > I have a vector of the element indexes that I want to 
> delete called index
> > so when I write myList[[index]] <- NULL to delete these 
> elements here is
> > what I get:
> > Error in myList[[index]] <- NULL : 
> >   more elements supplied than there are to replace
> > Isn't it possible to delete multiple elements?
> 
> If your index is a list of numerical indices as opposed to 
> names, then 
> just do
> 
> myList <- myList[-index]
> 
> the same way you'd delete elements from any vector.
> 
> If index is a vector of names, you need to convert it to 
> numbers, using 
> something like
> 
> indexnums <- which( names(myList) %in% index )
> 
> and then use myList[-indexnums].

If none of names(myList) are in the index vector then
indexnums will be integer(0) and myList[-integer(0)]
will be the same as myList[integer(0)] which has length
zero.  Hence you should check for length(indexnums)>0
before doing the subscripting.

Logical subscripting avoids that trap
    isNameInIndex <- names(myList) %in% index
    myList[!isNameInIndex]
(Read the '[' as 'such that').

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> 
> Duncan Murdoch
> 
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